Contact macroradiography characterization of doped optical fibers

ABSTRACT

The doping of the core of an optical fiber may be precisely characterized by cutting sample slices of the fiber by means of a focused ion beam (FIB) machine and by carrying out a contact radiography of the slices using a soft X-ray source. Maps of the distribution of the dopant ions in the glassy matrix of the optical fiber&#39;s core may be obtained by analyzing the contact radiographies at the electronic or atomic force microscope. A dopant concentration value per unit length of fiber may be determined by interpolating the results over a plurality of slices of different thicknesses.

FIELD OF THE INVENTION

The present invention relates in general to fabrication techniques ofoptical devices using doped optical fibers and more in particular tomethods of testing the doping profiles in the core of an optical fiber.

BACKGROUND OF THE INVENTION

Active optical fibers, typically doped with erbium, are more and moreoften used in communication systems. In particular the use of erbiumdoped active optical devices, and specially of erbium doped fiberamplifiers doped is rapidly growing because of the peculiar passband andgainband characteristics of these devices that favorably coincide withthe wavelengths of so-called third window, in the vicinity of 1.5 μm,which is more and more used in optical fiber communication systems.

One of the parameters that most affect doped optical fibers remains theconcentration and distribution within the vitreous matrix, generallysilicic, of dopant ions, in particular within the core of the opticalfiber.

The dopant ions act as active centers similarly to what happens inlasers and therefore their distribution inside the matrix is fundamentalin determining the performance of the active optical device.

In optical devices such as fiber amplifiers, the control of thedistribution of dopants in the glassy matrix of the core is even morecritical because in an optical fiber amplifier the smoothing (levelling)effect of the electromagnetic field due to optical feedback from cavitymirrors does not exist, whereas such effect does exist in lasers.

The article “Chlorine concentration profiles in silica fibers” by H.Hanafusa et al., ELECTRONIC LETTERS, Feb. 16, 1984, UK, vol. 20, no. 4,pages 178-179, and the article “Investigation of the structure ofperform materials and fiber-optical waveguides utilizing quartz glassdoped with germanium and boron”, by A. N. Gur'yanov et al., KVANTOVAYAELEKTRONIKA, Moskva, October 1979, USSR, vol. 9, no. 10, pages1238-1242, describe methods for characterizing the doping of the core ofan optical fiber and of a preform, respectfully, by x-ray microanalysis.These methods do not provide a direct and reliable distribution patternof dopant ions on a plane normal to an optical fiber axis.

The determination of the concentration profile, that is, asignificatively precise characterization of the doped optical fibercontinues to represent a serious problem in view of the difficulty toreliably make such a characterization and this represents a nonnegligible drawback for the commercial evolution of these technologies.

Until now the characterization is carried out in two differentfabrication steps of a doped optical fiber. A first characterization isundertaken on a rod or preform having a diameter of a few centimeters,before the material is drawn in a continuous small diameter fiber. Thecharacterization is made inductively, by exciting the material with alaser and measuring the intensity of the reemission.

However, the drawing process takes place at a temperature of about 800°C. and this inevitably determines a certain diffusion of the dopingmaterial during this manufacturing step. Therefore, it is necessary torepeat the characterization on the finished optical fiber, also in thiscase inductively, and to adapt consequently the length of the activeoptical fiber in order to attain the required gain value requested bythe specifications.

SUMMARY OF THE INVENTION

It is evident the need for a direct characterization method (noninductive) capable of precisely define the doping of an optical fibercore.

A method has now been found, and this is the object of the presentinvention, to solve this so far elusive problem of directly and reliablyanalyze the real distribution of dopant ions on a plane normal to theoptical fiber, capable of providing a reliable quality controlinformation on the doped optical fiber, as produced, and therefore itsintrinsic gain and passband characteristics.

It has now been found that is possible to obtain an optical image of thedopant distribution on a cut fiber portion by means of a soft X-rayContact Microscopy technique (SXCM).

According to a crucial aspect of the invention it is necessary toprepare sample slices of the optical fiber, thickness of which should besmaller than 20 μm, preferably 10 μm or even less.

With such a sample thickness, the soft X-ray contact microscopytechnique can attain reach a resolution of about 50 nm and, by use ofproper emission targets and/or of a filter having an appropriatespectral window, it is possible to enhance the contrast between thematrix of silicic glass and, specifically for the particular dopant ionsof the core.

According to the method of the invention, the distribution of therelevant dopant ions may be precisely mapped and their totalconcentration precisely estimated.

According to an important aspect of the invention, the cutting of sampleslices with a thickness (or length) in the vicinity of 10 μm, is carriedout using a Focused Ion Beam apparatus (FIB).

This type of equipment is described in the article: “Focused Ion Beam”,by Jon Orloff, Scientific American, Oct. 1991, pages 74-79.

These equipments are commercially available and they are used inmicroelectronics generally to perform local modifications or correctionsin already fabricated devices, by being able to perform precise cuts andeventually permit to deposit conductive layers of a specific geometry.These equipments are based upon the capacity to generate a beam ofstrongly accelerated ions that is made to impinge on the sample. The ionbeam, which can be precisely focused and guided by means ofelectrostatic lenses, incides the sample m -aterial being able todislodge (sputter) ions, electrons and atoms of bombarded surface.Therefore, an FIB may also be used similarly to an SEM, though withinferior resolutions.

The most important characteristic of an FIB is the intrinsic capacity ofremarkably restrict the scanning area of the ion beam on the samplesurface so to notably increase the density of ion-sample impacts andconsequently to attain a major transfer of energy to the sample from theincident particles on an extremely small area.

This energy generates a local microplasma and a consequent ablation ofsample material within the impact area of the beam, practicallyproducing obtaining a well defined cavity in the material beingbombarded.

The ion beam can be collimated in the order of ten nanometers so thatvery precise cuts may be performed.

The cutting depth depends solely on the total dose (number of ions persurface unit) of ions impacted onto the scanned zone.

Typically, the ions constituting the primary beam are simply ionizedions of gallium. Gallium possesses only two isotopes with about the sameisotopic abundance and is a liquid at ambient temperature. Thesecharacteristics permit to obtain a substantially monochromatic andhighly energetic beam, because the ionic mass is relatively large, about70 AMU, and a typical accelerating field is of about 25-30 KeV.

According to a further important aspect of the invention, with a plasmalaser source of soft X-rays capable of generating a beam of X-rays witha sufficiently ample diameter, several samples may be simultaneouslyradiographed, by arranging the samples over an X-ray photoresist layer,disposed on an appropriate support, for example on the bottom of asample holder which may have a cylindrical shape with a diameter and aheight of few millimeters.

A top cover of the cylindrical sample holder may be of silicon nitride(Si₃N₄), of about 0.3 to 3 mm thickness. The silicon nitride top coverdefines a suitable spectral window through which irradiate the samplesplaced onto the photoresist support of (e.g. the bottom of the sampleholder), thus performing a filtering of the soft X-ray (spectralwindow), which enhances the contrast between the matrix and the dopantsin the irradiated fiber samples.

The sample holder does not need to be evacuated.

A suitable source of soft X-rays may be a plasma laser developed at theRutheford Appleton laboratory (RAL), Central Laser Facility, (RAL_CLF),which may deliver an average X-ray power of 1 watt at 1 nm wavelengthinto 2π steradian, from a point source of a diameter of approximately 10μm. The source is constituted by a plasma laser 100 KrF operating inhelium at atmospheric pressure. The X-ray beam is focused to 1 mmdiameter for a fluence of 10¹² photons/second/mm², at 100 Hz laserrepetition rate, or focused to about 100 nm for a fluence of 10⁶photons/second/mm², at 100 Hz laser repetition rate.

The plasma source is excited by a picosecond excimer laser systemoperating at 248 nm wavelength and delivering a low diffraction beam oftrains of 16 pulses, each of 5 ps in length, separated by intervals of 2ns. The laser energy is of 350 mJ/pulse-train at a repeating rate of upto 100 Hz.

Another example of suitable source of Soft X-rays is the Asterix IVlaser system of the Max-Planck Institute fur Quentenoptik. In this casethe source is a high power iodine laser photolytically pumped at anemission wavelength of 1,315 um. The Asterix laser system is set in linewith a classical Master Oscillator Parametric Amplifier configuration.The pulse to be amplified is either produced by an acousto-opticallylocked oscillator generating pulses whose duration is less than ananosecond, or alternatively by a gain switched oscillator, deliveringnanosecond pulses. The amplifier chain consists of 6 amplifiers ofincreasing diameter and length. The final amplifier has an aperture of29 cm. The Asterix IV laser delivers pulsed length of 0.3 ns, a maximumoutput power of 4 TW, corresponding to an energy of 1.2 KJ.

While for the RAL-CLF it is necessary to position the samples at onlyfew millimeters from the source, when using a machine having the powercharacteristics of the Asterix IV it is possible to irradiate thesamples at a distance of several centimeters instead of few millimetersthus strongly reducing the penumbral blurring and thereby permittingassessments close to the theoretical limits of resolution.

A further possibility offered by a relatively powerful machine like theAsterix IV is to simultaneously radiate a plurality of sample holders offiber slices so to increase the production yield of the images and theefficiency and the test statistics.

Of course the spectral window of the X-ray beam may be suitably modifiedby selecting the nature of the target X emitter in order to enhance thecontrast between the matrix and the particular dopant of the core of thefiber.

The dopant ions, excited by the incident X-rays, absorb energy in a moremarked and definitely different manner than the surrounding matrix ofsilicic glass and in the photoresist layer sensible to X-rays amicroradiography is obtained which, after the development of thephotoresist, may be analyzed using an Atomic Force Microscope (AFM)capable of investigating the image (roughness) produced in the developedlayer of photoresist.

The critical parameters of this method of measuring the dose of absorbedX-rays are: the variation of the dopant distribution within the matrixof silicic glass, the sample diameter and its thickness.

Being the X-ray energy expressed in kiloelectrovolt, the resolution isin the order of 70 nm. Therefore, the dopant distribution may beeffectively mapped and its concentration estimated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 3 show soft X-rays absorption spectra for Erbium, SiO₂and Pr.

FIG. 4 is a schematic representation of the samples exposure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The diagrams showing the absorption for the different materials, thatare normally present in the core of the Erbium doped optical fiber, asshown in FIGS. 1, 2 and 3, provide an estimate of the difference betweenthe absorption properties of the respective materials existing in afiber's core.

FIG. 4 depicts the exposure to the X-ray of the photoresist layer,deposited on a substrate constituting the bottom of a cylindrical sampleholder, on the surface of which sample slices of optical fiber,sectioned by a Focus Ion Beams technique, are randomly placed.

The cylindrical sample holder or just the top cover of the cylinder maybe either of silicon nitride with a thickness ranging between 0.3 and 3millimeters or of any other material suitable to define a spectralwindow effective in enhancing the contrast of the microradiography.

The microradiography thus obtained may be either analyzed with anelectronic microscope or an Atomic Force Microscope to determine thedopant concentration based on an integrated measure of the dopantconcentration in the whole fiber slice.

Moreover, the method of the invention may also provide a goodapproximation of the concentration value for an hypothetical slice ofinfinitesimal thickness, that is, a dopant concentration substantiallyindependent from the thickness of the analyzed fiber slice, in otherwords a concentration value per unit length of fiber.

This is possible through a characterization undertaken on several samplefiber slices of different length (thickness) of the same fiber and byinterpolating the analytical results obtained on samples of differentthickness.

This capacity of the method of the invention provides in practice acharacterization instrument which allows to model the techniques andthermal treatments for preparing the preforms and drawing them in ahighly reproducible manner. In this way it is possible to define afabrication process which, differently from the known processes, doesnot require a final calibration test “ad fibram” to ensure that the gaincharacteristics of a manufactured fiber amplifier meet with the requiredspecifications.

What is claimed is:
 1. A method for analyzing the doping of an opticalfiber core, the method comprising the steps of: cutting a sample sliceof an optical fiber with a focused ion beam, the sample slice having athickness not greater than about 20 μm; placing the sample slice onto asurface of an X-ray photoresist layer disposed on a support; exposingthe sample slice on the photoresist layer to soft X-rays having awavelength between about 0.5 and 50 nm; developing the photoresist layerto produce a contact microradiography; analyzing the contactmicroradiography produced on the developed photoresist layer with anelectron microscope or an Atomic Force microscope; and obtaining a mapof the distribution of dopant ions in the optical fiber core.
 2. Themethod according to claim 1, further comprising the step of estimatingthe concentration of dopant ions in the optical fiber core.
 3. Themethod according to claim 1, wherein the step of exposing the sampleslice comprises exposing the sample slice to the soft X-rays through afilter of silicon nitride defining a spectral window of the X-rays. 4.The method according to claim 1, further comprising the steps of:cutting a plurality of sample slices of the optical fiber, the sampleslices having different thicknesses ranging between about 1-20 μm; andinterpolating dopant information derived from each sample slice todetermine a dopant concentration value per unit length of optical fiber.5. A method for analyzing the doping of an optical fiber core, themethod comprising the steps of: cutting a sample slice of an opticalfiber with a focused ion beam; disposing the sample slice on a surfaceof an X-ray photoresist; exposing the sample slice on the photoresist toX-rays; developing the photoresist; analyzing the developed photoresistto derive dopant information of the optical fiber core.
 6. The methodaccording to claim 5, wherein the step of developing the photoresistcomprises producing a contact microradiography, and further comprisingthe step of analyzing the contact microradiography produced on thedeveloped photoresist with an electron microscope or an Atomic Forcemicroscope.
 7. The method according to claim 5, further comprising thesteps of: obtaining a map of the distribution of dopant ions in theoptical fiber core; and estimating the concentration of dopant ions inthe optical fiber core.
 8. The method according to claim 5, wherein thestep of exposing the sample slice comprises exposing the sample slice tosoft X-rays through a filter of silicon nitride defining a spectralwindow of the X-rays.
 9. The method according to claim 5, wherein thesample slice has a thickness not greater than about 20 μm.
 10. Themethod according to claim 5, wherein the soft X-rays have a wavelengthbetween about 0.5 and 50 nm.
 11. The method according to claim 5,further comprising the steps of: cutting a plurality of sample slices ofthe optical fiber, the sample slices having different thicknesses; andinterpolating dopant information derived from each sample slice todetermine a dopant concentration value per unit length of optical fiber.12. A method of preparing a sample of an optical fiber, the methodcomprising steps of: disposing an optical fiber relative to a focusedion beam device; and cutting a sample slice of the optical fiber using afocused ion beam.
 13. The method of claim 12, characterized in that theions of the focused ion beam are single ionized ions of galliumaccelerated in a field of about 25 to 30 KeV.